Abstract

The Upper Permian carbonates of the second Zechstein evaporite (Stassfurt PZ2) cyclothem, the Main Dolomite (Ca2), constitute Poland’s most prolific oil play. An analysis of the Ca2 reservoir seismic response combined with wireline logging data from wells situated mostly on a lower slope and a toe-of-slope of the sulfate-carbonate platforms in Western Poland help to predict the quality and thickness of the reservoir. Low thickness of the strata (often below a limit of separability or even below a limit of visibility), changeability of their lateral and vertical reservoir quality, facial and mineralogical irregularity, and significant differences in the thickness inside lithostratigraphic units complicate a correct seismic interpretation. Correlation of well data with seismic data allowed us to understand a complex layout of reflectors. In particular, the appearance of an additional set of reflections on a slope and a toe-of-slope of the sulfate-carbonate platforms allowed us to predict a thicker Ca2 strata. The presence of a dim spot in the Z2 reflection created by the contact between the Older Halite and a thin layer of the Basal Anhydrite (A2) above the Ca2 reservoir (A2 and Ca2 are seismically treated as one unit) and the appearance of additional reflection or increase of its amplitude related to a base (Ca2b) of reservoir spread above the Werra Anhydrite indicate porous carbonate reservoir. To check conformity of a geologic model with the observed anomalous seismic image in the region of the Cychry South gas-oil reservoir, seismic modeling was carried out. We proved that a gas-saturated reservoir is relatively easy to identify. Distinguishing an oil-saturated reservoir from a water-saturated reservoir is practically impossible. One should be cautious when interpreting hydrocarbon-saturated zones because even insignificantly gas-saturated reservoir waters generate the same seismic image with the characteristic Z2 dim spot and amplitude amplification of the Ca2b.